And, by the way,  the "sphere in sphere" variometer has a big drawback: Its Q is quite OK at maximum inductivity but very poor at its minumum. Explanation is simple. Just assuming the resistance of the wire being constant, at low inducitivity the relation XL to R becomes very poor.

OK?

73 Ha-Jo, DJ1ZB


  "Rik Strobbe" <[email protected]> schrieb:

Hello Petr,

50-550kHz seems a "big shot".

If you want to tune (bring to resonance) the antenna using a variometer it would need a ratio of (550/50)^2 = 121.

The variometer I use on 500kHz has a range of 40-450uH, thus a ratio of 11. The variometer comes from an 500kHz marine transmitter and it would be rather complex to build a copy (mechanical).

Running low power you could use a parallel LC circuit. One end to the antenna and the other end to ground. The TX is connected to a tap at the coil, close to ground. You can tune the antenna to resonance by changing C (variable capacitor) and match to 50 Ohm by changing taps on the coil. I did that with success in the early days on 136kHz, but I could run only 30W power before the capacitor (plate distance 2mm) started arcing.

Now you will need a capacitor with a ratio of 121, but that is not so hard: most variable capacitors have a range of 20 or better and you can put some fixed capacitors in parallel (via switches).

73, Rik  ON7YD

At 09:26 2/04/2009, you wrote:

Hi all,

 

I want to make a small transmatch (RX, TX up to 10 Watts, or so) to tune LW 41 m (or smaller T-ant) in the range 50 ... 550 kHz.

I am not too good in theory... but I believe that the most efficient system is the popular loading coil (home made variometer, and taps to find 50 Ohms match).

The simple variometer (cylindrical coil in another cylindrical coil) is easy to make and works fine on 136kHz. However, it is possible to change inductance in the range about 1:2 or 1:3 only, not much better. Therefore the redudant inductance is too high to fetch the ant to resonance on 550 kHz.

Solution would be to make a more sophisticated variometer (best: sphere in sphere) to reach the ratio 1:10 or so.

Or, to use a different kind of network. Pi network, L network or T network. BTW I would also prefer to use a rotary switch and solder twenty condensers rather than to make twenty taps on a coil wound with litz wire...

 

How did you solve this yourself?

 

Thanks, 73, Petr, OK1FIG

 

 

 

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